1. Field of the Invention
The present invention relates to an image forming apparatus that masks noise generated therein.
2. Description of Related Art
Conventionally, to deal with noise generated by an image forming apparatus, a so-called “noise reduction technology” for reducing noise is employed. Noise reduction lowers the sound pressure level of noise, but does not completely solve problems of nearby workers perceiving “harshness” and “discomfort” caused by noise.
Aside from the Noise reduction technology, there has also been proposed a sound masking technology as a technology to lower noise (i.e., a technology to a level being less perceptible to nearby workers). Sound masking is a method taking advantage of a phenomenon (i.e., sound masking effect) in which perception of a sound at a certain level makes other sounds barely audible, and this method is mainly classified into frequency masking and temporal masking. More specifically, noise is superimposed with a sound (i.e., masker or masking sound) mainly similar in frequency band to the noise, so that the noise is made barely audible, thereby reducing harshness and discomfort.
As a conventional image forming apparatus applying the sound masking technology, there is a noise masking device described in Japanese Patent Laid-Open Publication No. 9-193506. This noise masking device includes a sound generator for generating masking sounds to mask operation sounds generated by various drive mechanisms, and a masking sound control unit for controlling the sound generator to generate a masking sound within a frequency range including the main component frequency of the operation sound. The masking sound control unit allows the masking sound to be generated within a frequency range between the lower and upper limits of a critical frequency band for the main component frequency of the operation sound.
In addition to the operation sounds of the drive mechanisms, the image forming apparatus generates various types of noise. Examples of such noise include: (1) collision noise made by a sheet material (e.g., paper) to be fed hitting a nip created by a rotor for feeding sheet materials; and (2) frictional noise made by the sheet material passing through a curved portion of a feeding path in the image forming apparatus.
The noise as specified in (1) and (2) above is suddenly generated at specific portions such as the nip and the curved portion of the feeding path. However, there are variations in precision of feeding sheet materials. For example, in the case where sheet materials are individually measured for the time of noise generation after the sheet materials leave a tray, variations in the time of noise generation among the sheet materials might be in the order of 100 milliseconds [ms], for example, due to delays of supply from the tray.
Furthermore, the rotor deteriorates over time, so that the generation time for the noises specified in (1) and (2) above can change in accordance with the duration of use of the rotor in addition to variations in precision of feeding sheet materials.
As can be appreciated from the foregoing, since the generation time for sudden noise can change, outputting a masking sound for such noise in accordance with uniform timing might not be satisfactorily effective.